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Appendix VI.                          1153

the fuel chamber lined with fire-brick and a jacket of sand, B the
hopper, and c the feeder; the air-fan D being used only when
starting. The fire-brick rests on a metal ring supported on two
segmental boxes E called superheaters, placed on the hottest part
of the fire, and mounted on a flat plate F right across the
producer. This plate has a central hole across which the fire-
bars G are placed. The jacket H forms a boiler to which water is
fed through the pipe j ; and the steam, together with air through
cock K, passes down the pipe L to the chamber M, whence it rises
by engine suction, and traversing the incandescent coal in A,
emerges by the pipe N as unpurified gas. Passing next through
the scrubbers p p, filled with loose coke down which water trickles
from the pipe R, the gas is freed from ammonia, which descends
as liquor into the box below, and overflows at T. Formerly a saw-
dust box was placed after the scrubbers to intercept any possible
tar, but the method being both unnecessary and prejudicial, has
been abandoned, any tar that might form in the producer
being at once converted into fixed hydrocarbons. The gas now
passes to the engine along the pipe u and through the expansion
box Y, the latter serving instead of the older gas-bag or equaliser.
When the engine stops, the cock v is opened to exhaust the gas
into the outer air : but very little is formed, as the suction action
is no longer present; and the plant is easily started again, the fuel
keeping hot a long time. The gas leaving the producer delivers
some of its heat to the baffle plates w, which act as conductor pins
to the water in the boiler.

Mond gas is obtained from common bituminous slack of very
low price, say 6s. to i is. per ton, according to district; and the
process is further cheapened by the complete recovery of ammonia,
as ammonia sulphate, to the extent of 6s. or Ss. per ton of fuel
burnt, reducing the actual working cost to less than ^-th of a
penny per B.H.P. hour. The gas is only suitable for driving
engines of large size, 250 H.P. and upwards, as the plant is both
large and expensive. In the smaller sizes the ammonia tower is
omitted. Referring to Fig. 1020, the heating and saturating
tower F is filled with tiles to distribute the hot water brought
from the tank L by the pump M, which is delivered by pipe R to
the top of the tower. After saturating the air from the blower E